The constant controls how many emails are sent per cycle but the software will still try to process as many cycles as it can in one go. The constant, in other words, is not going to work how you want it to. It's not intended to control the number of emails sent each time the task runs, it's meant to help control how many emails are sent each time we try to push the emails to the server - if your SMTP server is particularly slow, for instance, it may be able to handle 30 at a time but not 50.